The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Homogeneous charge compression ignition (HCCI) engines combust an air and fuel (A/F) mixture within cylinders to drive pistons and generate drive torque. HCCI engines may operate in a spark ignition (SI) combustion mode or an HCCI combustion mode to ignite the A/F mixture. Additionally, HCCI engines may operate in a mixed combustion mode that includes HCCI combustion with spark assist. For example, one of the SI combustion mode, the mixed combustion mode, and the HCCI combustion mode may be selected based on engine speed and/or engine load.
More specifically, the SI combustion mode ignites the A/F mixture in the cylinders using spark (e.g., from spark plugs). The HCCI combustion mode, on the other hand, ignites the A/F mixture by compressing the A/F mixture to a point of automatic ignition. In other words, an increase in temperature and/or pressure of the A/F mixture may result in ignition of the A/F mixture.
In the HCCI combustion mode, however, ignition may occur at several different locations at a time. In other words, the A/F mixture may burn nearly simultaneously. Therefore, the HCCI combustion mode performs close to an ideal combustion cycle (e.g., the Otto cycle). The near ideal combustion may result in increased fuel economy and/or decreased emissions compared to the SI combustion mode. The HCCI combustion mode, however, may be more difficult to control (and thus less stable) due to the lack of a direct initiator of combustion.